weic2523 — Science Release

9 December 2025

The NASA/ESA/CSA James Webb Space Telescope has confirmed the source of a super-bright flash of light known as a gamma-ray burst, generated by an exploding massive star when the Universe was only 730 million years old. For the first time for such a remote event, the telescope provided a detection of the supernova’s host galaxy. Webb’s quick-turnaround observations verified data taken by telescopes around the world that had been following the gamma-ray burst since its onset, which occurred in mid-March.

With this observation, Webb also broke its own record: The previous chart-topping supernova existed when the Universe was 1.8 billion years old.

*“Only Webb could directly show that this light is from a supernova - a collapsing massive star,” *said Andrew Levan, the lead author of one of two new papers in Astronomy and Astrophysics Letters and a professor at Radboud University in Nijmegen, Netherlands, and the University of Warwick in the United Kingdom. “This observation also demonstrates that we can use Webb to find individual stars when the Universe was only 5% of its current age.”

While a gamma-ray burst typically lasts for seconds to minutes, a supernova rapidly brightens over several weeks before it slowly dims. In contrast, this supernova brightened over months. Since it exploded so early in the history of the Universe, its light was stretched as the cosmos expanded over billions of years. As light is stretched, so is the time it takes for events to unfold. Webb’s observations were intentionally taken three and a half months after the gamma-ray burst ended, since the underlying supernova was expected to be brightest at that time.

“Webb provided the rapid and sensitive follow-up we needed,” said Benjamin Schneider, a co-author and a postdoctoral researcher at the Laboratoire d’Astrophysique de Marseille in France.

Gamma-ray bursts are incredibly rare. Those that last a few seconds may be caused by two neutron stars, or a neutron star and a black hole colliding. Longer bursts like this one, which lasted around 10 seconds, are frequently linked to the explosive deaths of massive stars.

Immediate, nimble investigation of the source

The first alert chimed on 14 March 2025. The news of the gamma-ray burst from a very distant source came from the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor), a Franco-Chinese telescope that launched in 2024 and was designed to detect fleeting events.

Within an hour and a half, NASA’s Neil Gehrels Swift Observatory pinpointed the X-ray source’s location on the sky. That enabled subsequent observations that would pin down the distance for Webb. Eleven hours later, the Nordic Optical Telescope on the Canary Islands in Spain was queued up and revealed an infrared-light gamma-ray burst afterglow, an indication that the gamma ray might be associated with a very distant object. Four hours later, the European Southern Observatory’s Very Large Telescope in Chile estimated the object existed 730 million years after the big bang.

“There are only a handful of gamma-ray bursts in the last 50 years that have been detected in the first billion years of the Universe,” Levan said.* “This particular event is very rare and very exciting.”*

Shockingly similar to nearby supernovae

Since this is the earliest, farthest supernova to be detected to date, researchers compared it to what they know in great detail - modern, nearby supernovae. The two turned out to be very similar, which surprised them.

Why? Little is still known about the first billion years of the Universe. Early stars likely contained fewer heavy elements, were more massive, and led shorter lives. They also existed during the Era of Reionisation, when gas between galaxies was largely opaque to high-energy light.

“We went in with open minds,” said Nial Tanvir, a co-author and a professor at the University of Leicester in the United Kingdom. “And lo and behold, Webb showed that this supernova looks exactly like modern supernovae.” Before researchers can determine why such an early supernova is similar to nearby supernovae, more data is needed to pinpoint tiny differences.

First look at supernova’s host galaxy

“Webb’s observations indicate that this distant galaxy is similar to other galaxies that existed at the same time,” said Emeric Le Floc’h, a co-author and astronomer at the CEA Paris-Saclay (Commissariat à l’Énergie Atomique et aux Énergies Alternatives) in France. Since the galaxy’s light is blended into a few pixels, making the galaxy look like a reddened smudge, what we can learn about it is still limited. Seeing it at all is a breakthrough.

The researchers have already laid plans to reenlist Webb in the international effort to learn more about gamma-ray bursts emitted by objects in the early Universe. The team has been approved to observe events with Webb and now have a new aim: to learn more about galaxies in the distant Universe by capturing the afterglow of the gamma-ray bursts themselves. “That glow will help Webb see more and give us a ‘fingerprint’ of the galaxy,” Levan said.

This research team observed supernova GRB 250314A with a rapid-turnaround Director’s Discretionary Time program (#9296).

More information

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

Image Credit: NASA, ESA, CSA, STScI, A. Levan (IMAPP), Image Processing: A. Pagan (STScI)

Links

• Release on ESA website
• Release on NASA website
• Science paper (A. Levan et al.)
• Science paper (B. Cordier et al.)

Contacts

Bethany Downer ESA/Webb Chief Science Communications Officer Email: [email protected]

ESA Newsroom and Media Relations Office Email: [email protected]